1. Field of the Invention
The present invention relates to a semiconductor thin film and a thin film transistor which have a high degree of crystal orientation, a method of manufacturing the same, and a manufacturing device of a semiconductor thin film.
2. Description of Related Art
A liquid crystal display device (LCD) as a typical example of existing thin panels is a power-saving, compact, and lightweight device. The LCD has been widely used for a monitor of a personal computer or a monitor of a portable information terminal device by utilizing the features. In recent years, the LCD has found its application in the field of television, and replaced conventional cathode-ray tube. However, the LCD has a problem that view angle and contrast are limited and high-speed response for displaying moving pictures is difficult. As a next-generation thin panel device that overcomes the above problem, an EL display device has been used. This device is a electroluminescence type display device that uses a light emitter such as an EL element for a pixel display portion. As described above, the EL display device is self-luminous with wide view angle, high contrast, and high-speed response as features that the LCD does not have.
In the display devices, a thin film transistor (TFT) is used as a switching element. As the TFT, a MOS TFT with a semiconductor thin film has been frequently used. The TFTs are classified into an inverted staggered type, a top-gate type, and the like, and the semiconductor thin films are classified into an amorphous semiconductor thin film and a polycrystalline semiconductor thin film. An appropriate one is selected there from in accordance with applications and performances of the display device. Many small panels employ a polycrystalline semiconductor thin film because an aperture ratio of a display region can be increased, and a TFT can be downsized. As a method of forming the polycrystalline semiconductor thin film, an amorphous semiconductor thin film is first formed on a silicon oxide film (SiO2 film) as a base film, and then applied with laser light to turn the semiconductor thin film into a polycrystalline film (see Japanese Unexamined Patent Application Publication No. 2003-17505).
As another known method, the polycrystalline semiconductor thin film is formed and then a TFT is manufactured. More specifically, a gate insulating film made of SiO2 or the like is formed on a polycrystalline semiconductor thin film to form a gate electrode. Next, impurities such as P (phosphorus) and B (boron) are doped into the polycrystalline semiconductor thin film through a gate insulating film to form source/drain regions. Further, the source/drain regions mean conductive regions containing impurities in the polycrystalline semiconductor thin film. The source region is connected to a source electrode, and the drain region is connected to a drain electrode later. Here, a region between the source/drain regions is a channel region. Subsequently, an interlayer insulating film is formed to cover the gate electrode and the gate insulating film. Then, a contact hole is formed in the interlayer insulating film and the gate insulating film to reach the source/drain regions of the polycrystalline semiconductor thin film. A metal film is formed on the interlayer insulating film and patterned to be connected to the source/drain regions formed in the polycrystalline semiconductor film to thereby form the source/drain electrodes. After that, a pixel electrode or EL element is formed to be connected with the drain electrode to thereby complete a TFT.
Further, in the case of forming a polycrystalline semiconductor thin film by a known method of applying a laser to an amorphous semiconductor thin film, crystal grains having a random size of about 0.2 to 1.0 um are arranged in the formed film. If a TFT is manufactured with a polycrystalline semiconductor thin film having various crystal grain sizes (crystal size), TFT characteristics vary because the number or size of crystal grains in a channel varies in accordance with a position of the TFT. For that reason, TFT characteristics vary. In the case of using a TFT having varying characteristics in a pixel or peripheral driving circuit, a voltage or a current written to each pixel thereof varies. The variations look like display unevennesses to a user, and display characteristics are deteriorated.
Thus, studies have been made of how to equalize randomly varying sizes of crystal grains and suppress variations in TFT characteristics. For example, “AM-LCD2000” (Y. Nakata, A. Shimoyama and S. Horita, p 265-268) describes that second harmonics of Nd:YAG laser (hereinafter referred to as “YAG-2ω laser”) are applied while a substrate is kept at 350° C. in a vacuum chamber, and then the substrate is rotated by 90° and irradiated with YAG-2ω laser again to thereby obtain crystal grains that are arrayed in a lattice shape at almost regular intervals. In the case of manufacturing a TFT with the thus-formed polycrystalline semiconductor thin film, the size or number of crystal grains in a channel can be equalized. Thus, variations in TFT characteristics are supposedly suppressed.
However, this method requires irradiation of the semiconductor thin film with the laser twice and thus is inappropriate to mass production. Further, the resultant crystal grain size is about 0.5 um that is substantially equivalent to a laser emission wavelength. Such a large crystal grain size causes deterioration of TFT reliability. This is because carriers accelerated by an externally applied electric field are repeatedly ionized by collision at a crystal grain boundary or in crystal grains to form an electron-hole pair. Alternatively, an internal defective level that is caused by enforced growth of crystal is increased, leading to deterioration in characteristics.